The long-term goal of this research is to understand prejunctional regulation of acetylcholine release from postganglionic parasympathetic nerves in the airways in health and in the presence of airway disease. It is hypothesized that during airway inflammtion, such as occurs in asthma and other airway diseases, there are changes in the response of nerves to prejunctional modulatory factors, and/or the milieu of mediators around parasympathetic nerves is altered so that there is either less inhibition or more facilitation of acetylcholine release. The proposed research will study factors that prejunctionally modulate acetylcholine release from horse trachealis muscle. The horse has been selected because its airways provide large amounts of muscle for concurrent studies of acetylcholine release and tension. In addition, the horse suffers from a chronic obstructive inflammatory airway disease known as heaves that has similarities to asthma and other obstructive lung diseases of humans. These diseases are characterized by airway obstruction, inflammation, and hyperresponsiveness, all of which are also feature of heaves. The mechanisms of hyperresponsiveness are not understood and may involve changes in prejunctional regulation of neurotransmitter release. Acetylcholine release in response to electrical field stimulation will be quantitated by HPLC and electrochemical detection. Concurrent measurement of the tension induced by field stimulation and exogenous acetylcholine will be made in adjacent strips of trachealis muscle. Indirect evidence of prejunctional effects derived from tension studies will be compared with measurements of acetylcholine release. The prejunctional role of muscarinic M2 receptors, sympathetic nerves, adrenoceptors, and prostanoids will be investigated in normal horses by examining the dose response to specific agonists and blocking agents. Airway obstruction will be induced in horses with heaves and the response to agonists and antagonists investigated and compared with normal horses. If differences in response are found, heavy horses will also be studied during clinical remission in order to demonstrate if the differences in prejunctional modulation are induced by airway inflammation.